In general, in electrophotography, the surface of a photoconductor drum is charged by a method such as corona discharge, followed by exposure using a laser etc. to form an electrostatic latent image. The formed latent image is developed with a toner so as to from a toner image. The formed toner image is transferred onto a recording medium to obtain an image with high quality. The toner used for formation of a toner image is typically a toner including toner particles (toner base particles) with an average particle diameter of 5 μm or larger and 10 μm or smaller produced by mixing a binder resin such as thermoplastic resin with components such as a colorant, a charge control agent and a release agent, followed by a kneading step, a pulverization step and a classification step. For the purpose of providing flowability or suitable charging performance for the toner particles, and/or for facilitating cleaning of the toner particles from the surface of the photoconductor drum, silica and/or inorganic fine particles such as those of titanium oxide are externally added to the toner base particles.
As to the toner particles in the toner described above, toner particles with excellent low-temperature fixability that can be satisfactorily fixed without heating a fixing roller where possible is desired from the viewpoint of energy savings, downsizing of equipment and so on. However, toner particles with excellent low-temperature fixability often tends to agglomerate when stored at high temperatures, and is susceptible to offset resulting from fusion of the toner particles to a heated fixing roller. This is because toner particles in the toner with excellent low-temperature fixability often contains a binder resin having a low melting point and a low glass transition point, as well as a release agent having a low melting point.
In view of the circumstances described above, for the purpose of improving the fixability of a toner to a sheet and offset resistance, a toner which includes toner particles containing a binder resin composed of a vinyl resin and an ionomer resin has been proposed.
For the toner particles in the above-described toner, however, fixability to a sheet and offset resistance are improved, but the level of improvement for fixability is inadequate. Therefore, for the above-described toner, further improvement of low-temperature fixability is desired. When an image with a low printing rate is formed over a long period of time, the toner particles in the above-described toner tend to be crushed if the toner particles are placed under stress over a long period of time as a result of stirring. In this case, a component such as a release agent contained in the toner particles tend to seep out onto the surfaces of the toner particles. When a component such as a release agent seeps out onto the surfaces of the toner particles, the storage stability of the toner particles at high temperatures is impaired.